fix value_to for variants

[grisumbras]

After we allowed exceptions to propagate through error_code-based conversions (when the user invoked value_to, rather than try_value_to), we inadvertently broke value_to for variants, because they relied on exceptions being caught by try_value_to. This change disables exception propagation for exactly one level of nesting.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.08%. Comparing base (3a7ee79) to head (9f85ed6).

Additional details and impacted files

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[cppalliance-bot]

Benchmark test results. More info at https://benchmark.cppalliance.org/jsonbenchmarks-pullrequests/999/pullrequest.html